LOFAR (the European LOw Frequency ARray) is the first of the new-generation radiotelescopes of the 21st century, that will culminate with SKA (Square Kilometer Array) after 2020. NenuFAR is a giant extension of LOFAR, that is also a powerful standalone low-frequency (LF) radiotelescope in the range 10-85 MHz. Its construction has started in 2014 in Nançay, supported by CNRS/INSU, Observatoire de Paris, Université d’Orléans/OSUC, and the FLOW consortium. NenuFAR is a compact antenna array (400 m in diameter) connected to the LOFAR receivers and to a local digitizer and beamformer (synthesizing narrow steerable 200 kHz beams in the sky up to a total instantaneous bandwidth of 150 MHz). We propose here to give NenuFAR the additional capability of a powerful LF imager with ~7’ resolution, by adding a realtime correlator, offline computing power, massive storage, and 6 small arrays (of 19 antennas each) within 3 km of the compact core. These extensions will considerably reduce the confusion noise limiting the imaging sensitivity of the instrument, down to about the thermal noise. The scientific topics that will greatly benefit from the use of NenuFAR in Standalone as an imager include: (a) in multi-frequency, long integration imaging: the detection of the cosmological HI signal from the “dark ages” to the reionization era through the "cosmic dawn", the systematic and sensitive search for yet unconfirmed LF radio signatures of exoplanets and star-planet plasma interactions (SPI), and the radio afterglows of high energy collapses generating gravitational waves and Gamma Ray Bursts ; (b) in snapshot imaging mode: the systematic sensitive search for fast radio transients in a broad field of view, including the prompt emission associated with gravitational waves, pulsar giant pulses, Fast Radio Bursts, and exoplanetary / stellar / SPI bursts ; Detection and study of ~10^5 galactic and extragalactic sources, with their diffuse LF emission, will also be made possible. For topics (a) & (b) the NenuFAR Radio Imager (NRI) will be in its frequency range the most powerful existing instrument before SKA (more powerful than LOFAR and the LWA), combining a high instantaneous sensitivity, very large total bandwidth, good angular resolution, and full polarization measurement capability. The proposed imaging capability, adding to its beamformer capability and its enhancement of LOFAR as a “Super Station” (LSS) will make NenuFAR a truly unique LF radio facility of the 21st century. In the SKA era, it will remain complementary to SKA in terms of spectral range and hemisphere. France has a strong expertise in development of imaging, calibration, deconvolution, and RFI mitigation algorithms for LF radioastronomy, that will be applied to and optimized for the NRI, preparing the exploitation of SKA. NenuFAR has received the official status of SKA pathfinder from the SKA office, for the experience it will bring in terms of hardware, algorithms, multimode operations (super-station / standalone beamformer / imager), and science preparation. The NRI will contribute to the organization and development of the French LF radio community – which started with the collective writing of a broad science case by 80 French and International scientists –, reinforcing its role and influence in Europe. The CNRS prospective in 2014 and the Haut Conseil TGIR in 2016 gave a high priority to the completion of NenuFAR (labelled “Infrastructure de Recherche” by the MESR), and the “Conseil Stratégique de Direction” of USN identified the NRI as the development of highest importance. The NRI project includes a Public Outreach part in the form of an Art Project integrated with the instrument and that will enhance its interest for the general public.